Process and apparatus for producing an intermetallic compound



/NVE/VTOR RALPH E @l MOTOR R. F. TRAMPOSCH AN INTERMETALLIC COMPOUND Filed Sept.

Nov. 19, 1968 PROCESS AND APPARATUS FOR PRODUCING F/G I United States Patent O 3,411,946 PROCESS AND APPARATUS FOR PRODUCING AN INTERMETALLIC CGMPOUND Ralph F. Tramposch, Sudbury, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Sept. 5, 1963, Ser. No. 306,950 30 Claims. (Cl. 117-201) This invention relates to semiconductor materials and more particularly to a thermoelectric technique for preparing semiconductor alloys from solution.

In the preparation of semiconductor alloys for use in transistors or lasers, it is desirable that alloy layers be of a controlled composition. Several attempts have been made in the past to obtain single crystal controlled alloy compositions, i.e., by promoting oriented growth of crystals upon a variety of single crystal substrate materials. A vapor evaporation technique was attempted many years ago, but the use of this technique has been limited due to the inability of producing uniform materials. Another method for growing semiconductor alloy layers by chemical decomposition from a Vapor phase has been developed, but this technique although finding some success, is limited because of the difficulty of producing materials that are free of impurities.

Accordingly, it is the principal object of this invention to provide a new technique for preparing semiconductor alloys.

It is a further object to provide a nonvacuumized technique for growing semiconductor alloys.

It is an additional object to provide a technique utilizing thermoelectricity to produce a semiconductor alloy.

It is a further object to provide a nonvacuumized technique for growing alloys composed of volatile materials or elements.

It is another object of this invention to provide a threeelement semiconductor alloy material which has a predetermined energy gap and dimensional spacing between atoms.

In accordance with the alloy preparation technique of this invention, compositions of semiconductor alloys are prepared by thermoelectrically controlling the amount of source material which is permitted to dissolve into and grow from a solution to form an alloy on a suitable substrate.

A source material is defined for purposes yof this invention as that material which comprises at least one of the materials which constitutes at least a portion of the final composition of matter or alloy.

Other objects and features of this invention will become apparent from the following description taken in connection with the drawings, wherein:

FIG. 1 is a sectional view of an apparatus utilized in carrying out the principles of the present invention; and

FIG. 2 is a schematic diagram of the electrical portion of the apparatus which is used to control the composition of the alloy prepared in accordance with this invention.

Referring now to the drawing, and more particularly to FIG. 1 thereof, there is shown at 10 a bottom tube seat constructed of a bronze material. A top tube seat, also of bronze, is shown at 12. Sealed between seats and 12 is a quartz tube muflle 11. Sealing of the seats and mutlie is accomplished by the tightening of sealing jackets 13 and 15, and the simultaneous forcing of O- rings 14 and 16 into contact with the mule 11. A rotatable assembly is shown at 17. The assembly is supported by the bottom seat 10. Assembly 17 comprises a graphite electrode 18 having mounted thereon an inner steel jacket secured by a steel pin 20. An outer steel jacket 24 is 3,411,946 Patented Nov. 19, 1968 shown mounted on the inner jacket. The inner jacket is used to position a quartz crucible 21 above the electrode 18 in order to permit a seed or substrate 30 to be positioned on a top surface 23 of the electrode 18. The substrate 30 is rigidly positioned between the electrode top surface 23 and the quartz crucible 21. A thermoelectric expansion gasket 22 is mounted between crucible 21 and jacket 24.

Graphite electrode 18 has a gear 25 mounted on one end to lpermit the lassembly 17 to be rotated, thereby permitting the substrate 30 to rotate to obtain uniformity in crystal growth. A second -gear 26 drives gear 25 by way of a shaft 27 which is coupled to a motor 28. O- rings 29a and 29b seal the moving electrode 18 in the seat 10.

A silicate housing 40 is shown mounted between the outer jacket 24 and the top seat 12 by a quartz insulator 41. A quartz ring disc 42 cutout in the shape of a washer is shown positioned within the housing 40, and on top of the crucible 21. The disc 42 permits a portion of a solvent 35 to be held in position above the substrate 30. A first source -of material 33 and a second source of material 34 in the shape of half discs are mounted in contact with the solvent 35 interface. The materials 33 and 34 are separated by a quartz rectangularly shaped insulator 36.

Three openings in top seat 12 are shown, each of which are lined with quartz insulating jackets 44, 45 and 46. A first electrode 50 passes through jacket 44 to enter a cavity 56 in housing 40. A solvent 31 is used to make good electrical contact between the material 33 and the electrode 50. A second electrode 51 passes through the jacket 46 to enter a cavity 58 in housing 40. A solvent 32 is used to make electrical contact with source material 34. A thermocouple 53 passes through the jacket 45 to enter a cavity 54 in housing 40. Thermocouple 53 is used to measure the temperature in the vicinity of the materials 33, 34 and substrate 30 during the alloy growing process. Electrical contacts are made to electrodes 18, 50 and 51 by leads 63, 62 and 61, respectively. Leads and 66 connect thermocouple 53 to external temperature metering devices.

A conventional resistance heating apparatus is shown at having electrical leads 73 and 74. The heating apparatus 70 is positioned around the mutile 11 in order to keep the materials 33, 34, solvent 35, and substrate 30 at a proper system -operating temperature so that rapid dissolution of materials 33 and 34 can take place, and further, to insure that the solvent remains in a liquid form. An intake gas tube 76 and valve 75 is mounted on top seat 12. The valve is used to permit an inert gas to enter the apparatus. A second exhaust valve 77 and tube 78 is mounted on bottom seat 10 to permit a constant flow of the inert gas.

Referring to FIG. 2, there is disclosed a schematic diagram of a coupling arrangement to control thermoelectric heating of the materials 33 and 34 and the solvent 35. A current supply 80 is shown providing a current z' toelectrode 18. The flow of current is in a direction such that the current enters electrode 18 and first travels through the substrate, then the solvent, and last through the source materials. This direction of current flow provides for cooling at the interface between the substrate and the solvent and heating at the interface of the solvent and the source materials. A variable resistor 82 is connected to electrode 51 to control the amount of current flowing through a path comprising substrate 30, solvent 35, and source material 34. A second variable resistor 83 is connected to electrode 50 to control the amount of current flowing through a path comprising substrate 30, solvent 35, and source material 33. By adjusting these resistors,

the composition of the alloy to be formed is controlled. Composition of the alloy is controlled by the -current density which in turn determines the amount of materials 33 and 34 dissolved at Athe interface of the solvent 35 due to thermoelectric (Peltier effect) heating at a solid to liquid interface. It is to be noted that the material that dissolves enters the solvent solution and crystallizes onto the substrate due to cooling taking place at the interface between the liquid solvent 35 and the solid substrate 30.

Referring once again to FIGS. 1 and 2, a description will be given for applying this invention to the preparation of a gallium arsenide phosphide alloy. By way of example, a single crystal of gallium arsenide oriented along its (111) crystallographic axis and having a surface area in the order of 1/2 square centimeter at its interface with a solvent 35, is chosen as a suitable substrate 30 for the preparation of this alloy. A substrate comprised of at least a portion of the alloy to be formed is preferred in order to limit the possibility of impurities forming in the alloy to be grown upon a surface of the substrate. The crystallographic axis orientation is chosen in this instance to obtain a grown single crystal which will exhibit a (lll) crystallographic axis orientation. Although this axis is chosen, it is to be understood that other orientations of crystal growth, for example, along the (101) axis, is obtainable by using a substrate oriented along its (101) crystallographic axis.

Gallium (Ga) is chosen as a solvent 35, in this instance, since it is a liquid at the temperatures used in the preparation of the alloy. Furthermore, the use of a solvent comprised of one of the elemental materials of the alloy to be formed also prevents the possibility of impurities entering the alloy. Gallium is also chosen as solvents 31 and 32 in order to permit a good electrical contact to be made by the electrodes 50 and 51 and source materials 33 and 34, respectively. The quartz disc of this invention, 42, has a thickness dimension in the order of .020 inch and has a quartz-free area in the disk of approximately 1/2 square centimeters. The depth of the gallium solvent is slightly greater than .020 inch, which is the thickness dimension of the disc 42. A first solid source material 33 of gallium arsenide (GaAs) is used as disc 33. A second source material of gallium phosphide (GaP) also comprises material 34. Each of the materials 33 and 34 has a surface area in contact with the solvent 35 in the order of between Ms square centimeter and 1A square centimeter. An inert gas, such as 95% nitrogen and 5% hydrogen, is continuously pumped through the apparatus via intake valve 75 and exhaust valve 77. The materials 33 and 34, the solvent 35, and the substrate 30 are heated by the resistance heating apparatus 70 to a temperature above 800 C. but below l000 C. At these temperatures gallium 35, 31 and 32 is in a liquid form and the substrate Ga As and the source materials Ga, As, and GaP are in a solid form. The substrate 30 is continuously rotated by motor 28 which in turn rotates the assembly 17. This permits a homogeneous crystal growth to form at the interface between the solvent 35 and the substrate 30.

The current z' shown in FIG. 2 is permitted to flow into electrode 18. This current i passes through the substrate 30, the solvent 35, and divides between the two source materials 33 and 34. The division of the current i between the two source materials 33 and 34 is controlled by the variable resistors 82 and 83, connected to electrodes 51 and 50, respectively. By controlling the amount of current and the current density in this manner, the percentages of the chemicals As and P in the grown alloy can be controlled. Current densities in the order of 40 amps. per square meter were utilized in the preparation of gallium arsenide phosphide alloys. The flow of current i at the interface between the source materials 33 and 34, and the solvent 35, produces a thermoelectric heating effect or phenomena. This effect is generally known as the Peltier effect. The heating at the interface of the solvent and the sources of materials causes the materials to dissolve and saturate the solvent to a predetermined degree dependent upon the current density passing through each of the materials. The amount of current density, therefore, controls the dissolution rate of the alloy material components and consequently the composition of the grown alloy crystal layer. Growth on the substrate 30 takes place because of the thermoelectric phenomena, known as the Peltier effect, which causes cooling to take place at the interface of the liquid gallium 35 and the solid substrate 30. This cooling causes crystallization of the materials which were previously dissolved into solvent.

Thus, a technique has been described for growing a gallium arsenide phosphide crystal alloy material which is today finding many uses in laser applications. The technique described herein permits the growth of semiconductor alloys having predetermined energy band gaps since the energy band gap is a function of the alloy composition of the material. Additionally, alloys having predetermined atomic dimensions between atoms can be grown. Materials having predetermined frequency characteristics can be obtained using the technique described herein. These materials are particularly suitable for use as crystals in laser applications. Furthermore, it is to be noted that strictly from an impurity standpoint, much higher purity alloys can be obtained by employing this method since only those elements which are involved in the preparation of the alloy material are present.

The technique of the present invention is also suitable for growing other alloys from a solid solution such as an indium arsenide phosphide alloy using an indium solvent and operating at a solution temperature of approximately 550 C. Another alloy material of a germanium .gallium arsenide compositon could also be epitaxially grown using gallium as a solvent and operating at a temperature of approximately 550 C. Other alloys could comprise silicon gallium phosphide using gallium as a solvent and operating at a temperature of approximately 1000 C. and aluminum arsenide phosphide material -using aluminum as a solvent. In additon, the technique of this invention is suitable for epitaxially growing two-element crystals by rusing a single source material of the crystal composition desired. By changing the crystallographic axis orientation of the substrate, a two-element crystal having any desired crystal lattice orientation can be obtained. This also holds true of three or more elemental crystal alloy structures. Accordingly, it is desired that this invention not be limited except as defined by the appended claims.

What is claimed is:

1. A nonvacuumized process for producing an intermetallic compound comprising passing a current ow through at least one source material capable of being dissolved and thereafter recrystallized, a solvent forming a solid to liquid interface with said material, and a substrate forming a liquid to solid interface with said solvent, said current producing a thermoelectric heating effect to dissolve said source material into said solvent at said solid to liquid interface and a thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating on said substrate.

2. A nonvacuumized process for producing an intermetallic compound comprising passing a flow of current through at least two metallic source materials capable of being dissolved and thereafter recrystallized, a solvent forming a solid to liquid interface with both of said materials, and a substrate forming a liquid to solid interface with said solvent, said ow of current producing a thermoelectric heating effect to dissolve said source materials into said solvent at said liquid to solid interface and a thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

3. A process in accordance with claim 2 wherein said solvent comprises an element found in at least one of said source materials.

4. A process in accordance Iwith claim 2 including simultaneously heating said materials to a temperature above the melting point of said solvent.

5. A process in accordance with claim 2 including individually controlling the flow of current through each of said materials to control the percentage of each source material contained within the grown alloy.

6. A process in accordance with claim 2 'wherein at least one of said source materials comprises a volatile substance.

7. A process for growing an intermetallic compound comprising passing a ow of current through a rrst layer comprising a first multi-element source material of GaAs and a second multi-element source material of GaP insulated from each other, a solvent layer forming a solid to liquid interface with said source materials, and a substrate layer forming a liquid to solid interface with said solvent, said current producing a thermoelectric heating effect to dissolve said source materials into said solvent at said solid to liquid interface and a thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

8. A process in accordance with claim 7 including an additional step of controlling the ow of current passing through each of said source materials to control the composition of said alloy.

9. A process in accordance with claim 7 wherein said solvent comprises Ga.

10. A process in accordance with claim 8 includingy an additional step of heating said materials, solvent and substrate to a temperature above the melting temperature of said solvent.

11. A process for producing an intermetallic compound comprising passing a flow of current through a combination of a first layer comprising a first multi-element source of InAs and a second multi-element source of InP insulated from each other, a solvent layer forming a solid to liquid interface with said source materials, and a substrate layer forming a liquid to solid interface with said solvent, said current producing a thermoelectric heating effect to dissolve said source materials into said solvent at said solid to liquid interface and a thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

12. A process in accordance with claim 11 wherein said solvent comprises In.

13. A process in accordance iwith claim 11 including an additional step of controlling the flow of current passing individually through each of said source materials to control the composition of said compound.

14. A process in accordance with claim 12 including an aditional step of heating said source materials, solvent and substrate to a temeperature above the melting temperature of said solvent.

15. A process for producing an intermetallic compound comprising passing a ow of current through a combination of a first layer comprising a source material of Ge and a multi-element source material of GeAs insul-ated from each other, a solvent layer forming a solid to liquid interface with said source materials, and a substrate layer forming a liquid to solid interface with said solvent, said current producing a. thermoelectric heating effect to dissolve said source materials into said solvent at said solid to liquid interface and a thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

:16. A process in accordance with claim 15 including an additional step of controlling the flow of current passing individually through each of said source materials to control the composition of said compound.

17. A process in accordance with claim 16 including an additional step of heating said materials, solvent and substrate to a temperature above the melting tmeperature of said solvent.

18. A process in accordance with claim 15 wherein said solvent comprises Ga.

19. A process for producing an intermetallic compound comprising passing a flow of current through a combination of a first layer comprising a first source material of Si and a multi-element source material of GaP insulated frorri each other, a solvent layer forming a solid to liquid interface with said source materials, and a substrate layer forming a liquid to solid interface with said solvent, said current producing a thermoelectric heating effect to dissolve said source materials into said solvent at said solid to liquid interface and a thermoelectric cool.. ing effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

20. A process in accordance with claim 19 including an additional step of controlling the flow of current passing individually through each of said sounce materials to control the composition of said alloy.

21. A process in accordance with claim 20 including an additional step of heating said materials, solvent and substrate to a temperature above the melting temperature of said solvent.

22. A process in accordance with claim 19 wherein said solvent comprises Ga.

23. A process for producing an intermetallic compound comprising passing a flow of current through -a rst layer comprising a multi-element source material of AlAs and a multi-element source material of AlP insulated from each other, a solvent layer forming a solid to liquid interface with said source materials, and a substrate layer fonming la liquid to solid interface with said solvent, said current producing a thermoelectric heating effect to dissolve said source materials into said solvent at said solid to liquid interface and Ia thermoelectric cooling effect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

24. A process in accordance with claim 23 including an `additional step of controlling the ow of current passing through each of said source materials to control the composition of said alloy.

25. A process in accordance with claim 24 including an additional step of heating said materials, solvent and substrate to a temperature above the melting temperature of said solvent.

26. A process in accordance with claim 23 wherein said solvent comprises Al.

27. A process for producing an intermetallic compound comprising providing a current ow through at least one source of material selected from the group consisting of Ge, Si, GaAs, GaP, InAs, InP, AlAs, and AlP, a solvent forming a solid to liquid interface with said source, said current producing a thermoelectric heating effect to dissolve said source Imaterial into said solvent at said solid to liquid interface and a thermoelectric c001- ing elect at a solvent substrate interface spaced from said solid to liquid interface to form an intermetallic compound coating of said substrate.

28. An apparatus for an intermetallic compound comprising means for placing iat least one source material in contact lwith a solvent, means for placing a substrate in contract with said solvent, and means for providing a current flow through said source material, said solvent land said substrate to create a heating effect to dissolve said source material into said solvent at said solid to liquid interface and a thermoelectric cooling effect yat a solvent substrate interface spaced from said solid to liquid inter- 7 8 face to form an intermetallic compound coating of said References Cited Substrate. NI T PATENT 29. An apparatus in accordance with claim 28 includ- U TED STA ES S ing means for heating said solvent to a temperature 2946700 7/1960 Day 117-93 3,198,606 8/1965 Lyons 23-301 above its melting point. 5

30. An apparatus in accordance with claim 28 includ- 3,211,547 10/1965 Jarrett et 3L 23- 305 ing means for rotating said substrate to obtain 4a homogeneous Compound WLLIAM L. JARVIS, Przmary Examzner. 

1. A NONVACUMIZED PROCESS FOR PRODUCING AN INTERMETALLIC COMPOUND COMPRISING PASSING A CURRENT FLOW THROUGH AT LEAST ONE SOURCH MATERIAL CAPABLE OF BEING DISSOLVED AND THEREAFTER RECRYSTALLIZED, A SOLVENT FORMING A SOLID TO LIQUID INTERFACE WITH SAID MATERIAL, AND A SUBSTRATE FORMING A LIQUID TO SOLID INTERFACE WITHSAID SOLVENT, SAID CURRENT PRODUCING A THERMOELECTRIC HEATING EFFECT TO DISSOLVE SAID SOURCE MATERIAL INTO SAID SOLVENT AT SAID SOLID TO LIQUID INTERFACE AND A THERMOELECTRIC COOLING EFFECT AT A SOLVENT SUBSTRATE INTERFACE FROM SAID SOLID TO LIQUID INTERFACE TO FORM AN INTERMETALLIC COMPOUND COATING ON SAID SUBSTRATE. 